In traffic supervision and accounting of operator-assisted calls, it is necessary to collect and correlate data from switching nodes at different locations, for example, at a switchboard and at other points of an exchange. In particular, it is advantageous for accounting purposes to know the identity of a plug connection used for establishing a call, the identity of the switchboard and the identity of the trunk engaged in the implemented call connection (datum from the exchange).
Devices for detecting and correlating information coming from different points of a call connection, particularly information relating to the identity of a calling subscriber and to the identity of a called subscribed, are known in the telephone switching art. Any stage marker of an indirect-control exchange may serve as an illustration.
However, such devices discover only the end station of an imminent connection which is then set up upon the finding of a free path; they are useless in the case of a manually implemented connection such as that effectuated at a switchboard. Furthermore, conventional line identifiers employed at automatic exchanger cannot operate if the same line is used by many connections simultaneously, i.e. in the event of a conference among a plurality of operators or of a connection transfer from one operator to another.